A major industry has developed in the protection of the threads of oil field tubular goods to prevent them from corroding during the period between manufacture and use, because of the high cost of such tubular goods and because of the importance of being sure that there are no defects in the threads which could cause them to leak after the pipe has been installed in a well. Oil well tubing often must withstand a pressure of hundreds or even thousands of pounds per square inch. A leak after the tubing is installed means the tubing must be pulled and repaired or replaced, at a cost of perhaps hundreds of thousands of dollars.
Pipe thread corrosion may be ordinary oxidation, or rust, or it may be caused or supplemented by micro-organisms which feed on various materials on the surface of the thread and produce an acid which causes pitting of the threads. A variety of means have been used to prevent such corrosion, with questionable success. Commonly, API (American Petroleum Institute) Bulletin 5A2 pipe dope has been used, although it is low in corrosion inhibiting properties. This pipe dope is designed to be a thread compound, with lubricating and sealing properties. It is a thick grease-based material which contains lead and other filler materials to seal the helical passageway in the round profile threads commonly used on oil field tubular goods. Another material which has come into use is a wax-based material sold under the trademark KENDEX. Other, lighter materials, such as a light oil, are sometimes used if the pipe is to be used within a day or two of the time it is threaded. Such corrosion inhibitors are applied to the threads as soon as the pipe is threaded, in an effort to prevent flash rust which can occur in a very short time. Such flash rust is particularly objectionable on the high-precision threads known as "premium" threads which depend on a metal to metal seal.
After the corrosion inhibitor has been applied to a newly machined thread, the thread is further protected by screwing on a cover known as a thread protector. Thread protectors may be made of metal or plastic, or combinations of metal and plastic, and are constructed to protect the threads against impact damage when the pipe is accidentally dropped or bumped. Many thread protectors, aptly called "dust covers" in the field, are loose fitting and are of no value in keeping moisture away from the threads. Some, however, are cup-shaped, i.e. they include an end cover to close the end of the pipe, and some are also snug fitting and include moisture seals in an effort to improve corrosion protection.
In the manufacture of threaded tubular goods, the threading machines use a water-based cutting fluid. After threading, the cutting fluid is either wiped off or blown off with air, the "corrosion inhibitor" is immediately applied, and thread protectors are put on. The pipe is then put in a pipe storage yard, usually exposed to the elements, until it is needed. The pipe may remain in the storage yard for a year or more, depending upon demand. It is during this period that protection of the threads against corrosion is most critical.
However, the oil, grease or wax-based materials which have previously been used to inhibit corrosion have some major drawbacks. The manufacturing operation, and even the methods used to clean the cutting fluids from the threads, necessarily leave some water on the threads, as well as some organic and sulfur compounds, and often microbes which can feed on the organic and sulfur-containing materials and excrete acids. The grease and wax-based materials previously used do not absorb or remove the water, but instead enclose it on the surface of the threads, leaving oxygen and microbes in contact with the threads to do their damage.
Because of this, if the pipe, with corrosion inhibitor and thread protectors in place, stays in a storage yard for more than about 60-90 days, it is desirable to inspect the threads to be sure that corrosion has not begun. This is done by removing the thread protectors, cleaning off the corrosion inhibiting material with a solvent, steam or some mechanical means, inspecting the threads, reapplying corrosion inhibitor, and either applying new thread protectors or cleaning and reapplying the ones previously used. As long as the pipe is in the yard, this process should be repeated every 60-90 days to ensure against thread corrosion.
This inspection process is expensive in the cost of the corrosion inhibitor and thread protectors, but more importantly in the cost of manpower for cleaning and retreating the threads. Moreover, every time the lead-containing API dope is cleaned off, there is a contamination problem which must be dealt with. It has been estimated that as much as two million pounds of lead are deposited on the ground each year through this process alone. Kendex and other inhibitors previously used are cleaned off with solvents, such as naphtha, diesel fuel or other petroleum-based materials which flow onto the ground and cause a contamination problem.
When the pipe is finally sent to a rig for installing in a well, it is necessary to again remove the thread protectors, clean the threads, inspect for corrosion, apply a thread sealant, and then make up the joints for installation in the well. Such materials as the API pipe dope and Kendex are not compatible with the newer anaerobic sealants used as thread joint compounds, such as those disclosed in U.S. Pat. No. 4,813,714, one embodiment of which is manufactured by Loctite Corporation of Newington, Conn., and sold under the trademark SEALLUBE. It is necessary to remove the grease or wax based material and thoroughly clean the threads before this type of sealant is used.
Others have recognized the problem with the materials commonly used as corrosion inhibitors on oil field tubular goods, and have proposed solutions. Levesque, in an article entitled "Rust-inhibition fluids protect OCTGs in storage", published in WORLD OIL, March, 1985, discussed the use of tin or zinc plating or phosphate coatings for this application. He also pointed out the desirability of non-metallic snug fitting thread protectors with moisture seals to reduce corrosion.
Vapor phase corrosion inhibitors are well-known for the protection of steel from corrosion during shipping and storage. Such inhibitors are described in U.S. Pat. Nos. 2,643,177 and 3,779,818 and in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 7, pp. 137-138. These inhibitors produce a vapor which precipitates a very thin film which is adsorbed on exposed surfaces. Known vapor phase inhibitors include amine salts with nitrous or chromic acids, amine salts with carbonic, carbamic, acetic and substituted or unsubstituted benzoic acids, organic esters of nitrous, phthalic or carbonic acids, primary, secondary and tertiary aliphatic amines, cycloaliphatic and aromatic amines, polymethylene amines, mixtures of nitrites with urea, urotropine and ethanolamines, nitrobenzene and 1-nitronaphthalene. Two common vapor phase inhibitors are dicyclohexylamine nitrite and cyclohexylamine carbonate. However, both of these inhibitors have some degree of toxicity, and various non-toxic proprietary compositions have therefore been produced and are in use to provide corrosion protection. One of those compositions is sold under the trademark Cortrol VCI, and is manufactured by Corless North-America, Inc. of Stanford, Conn. Cortrol VCI is a water soluble mixture of sodium benzoate and approximately 15% to 25% alkylated sodium benzoate, containing trace amounts of water and amine salts. Such a material may be made by the partial oxidation of coal tar distillates which contain mixtures of toluene, xylene and other alkylated benzenes to form benzoic acid and alkylated benzoic acid, followed by reaction with sodium hydroxide or sodium carbonate.
The vapor phase inhibitors are intended to be used dry, but many of them will continue to provide some corrosion control when dissolved in water. Generally, however, when the surface to be protected is immersed in water, as in steel tanks or boiler tubes, liquid phase inhibitors, such as the water soluble inorganic and organic salts which create a passive surface on the metal, are used. Such liquid phase corrosion inhibitors are disclosed, for example, in U.S. Pat. No. 2,550,997, and include the nitrite salts of alkali metals, alkaline earth metals and aromatic, aliphatic and heterocyclic amines which are not subject to auto-decomposition at ambient temperature. Benzoic acid and its salts, such as sodium benzoate, potassium benzoate and ammonium benzoate, are also effective liquid phase inhibitors. Because of cost factors and availability, sodium benzoate, sodium nitrite and potassium nitrite are generally preferred.